Vesicular-arbuscular mycorrhizae (VAM) are present in most terrestrial ecosystems and play a major role in community structure and function. However, their role in primary succession remains poorly understood. Two greenhouse studies examined the role of VAM in Mount St. Helens pioneer species under three nutrient regimes and four competitive scenarios. Nutrient levels were complete, complete without phosphorus (−P), and tap water (very low nutrient levels). In tap water a negative effect from VAM colonization was observed perhaps due to parasitic action of the VAM fungi. A weak but apparent benefit from VAM occurred in the −P treatment since plants in the −P treatment were usually not less in biomass than those in the complete nutrient treatment and VAM colonization levels were greater in the −P treatment. VAM colonization was more beneficial to plants under the complete nutrient treatment than under the tap water treatment. VAM assisted the facultatively mycotrophic Hypochaeris radicata in competition with the non-mycotrophic Carex mertensii. Lack of VAM improved the competitive ability of Carex mertensii when in competition with facultatively mycotrophic species. However, VAM did not significantly influence competitive outcomes between facultatively mycotrophic species.